RADIOGENIC ISOTOPE ANALYSIS OF BASEMENT ROCK AS A POTENTIAL SOURCE METALS IN MISSISSIPPI VALLEY-TYPE ORES, SOUTHERN MID-CONTINENT, U.S

Mississippi Valley-Type (MVT) ore deposits are an economically viable source of lead (Pb) and zinc (Zn). Many mining districts of these ores exist around the globe, but world-renowned deposits lie in the mid-continent region of the United States. The current ore genesis model suggests mineralizing fluid containing Pb and Zn transported through vast hydrothermal networks to deposit these sulfide ores during large-scale tectonic events.

The Tri-State, Northern Arkansas, Central Missouri, and Southeast Missouri mining districts are the relevant MVT deposits for this study. Published Pb-isotope data suggest that the ores have similar metal sources. However, these source(s) are not well constrained. Precambrian basement, as well as Paleozoic organic-rich shales proximal to the MVT mining districts, have been hypothesized as potential sources of metals.

This study will test the hypothesis that basement rock supplied the Pb and Zn necessary to form these sulfide MVT ores. The analyzed samples were gathered from cores and cuttings of the deepest penetrating wells in Arkansas, Oklahoma, and Kansas. The geochemical preparation of the samples has been carried out in the Class 100 clean lab at the University of Arkansas. The prepared samples have been analyzed for trace element compositions and radiogenic isotope ratios (Pb) via Quadrupole ICP-MS and MC ICP-MS, respectively. The Pb isotope ratios of basement rocks are compared to those of known MVT ores. Similar trending isotopic signatures may imply the basement rocks are a source of metals in the southern mid-continent MVT deposits.

Preliminary data from nine basement samples are available. The ²⁰⁸Pb/²⁰⁴Pb signature for the Northwest Arkansas samples range from 37.83 to 39.20, the ²⁰⁷Pb/²⁰⁴Pb from 15.46 to 15.60, and the ²⁰⁶Pb/²⁰⁴Pb from 17.81 to 19.81. The Northern Kansas samples yield ²⁰⁸Pb/²⁰⁴Pb ratios between 37.15 and 37.86, ²⁰⁷Pb/²⁰⁴Pb between 15.52 and 15.60, and ²⁰⁶Pb/²⁰⁴Pb between 17.93 and 18.98. On the thorogenic and uranogenic diagrams, these samples plot along a lesser radiogenic field compared to ore signatures. Further analysis of an expected fifty samples will provide a regional scale picture of basement Pb signatures of the entire Southern Mid-continent and will further clarify the possibility of a basement metal source for MVT deposits.